1. Field of the Invention
This invention relates to computer systems and more particularly to computer systems where Input/Output (I/O) devices attached to the central processing unit (CPU) are serviced on a cycle steal basis and still more particularly to such computer systems where at least one of the I/O devices is serviced in a dedicated mode where the CPU grants as many successive cycle steals as the I/O device requires and where the I/O device operating in the dedicated mode relinquishes cycle steals to another I/O device.
2. Description of the Prior Art
I/O devices operating in a dedicated mode on a cycle steal basis often have a data rate which is different from the rate at which data storage access cycles are available. Hence, data transferred from storage to the I/O device via the CPU may be lost if the I/O device cannot use it fast enough. The prior art approach to the solution of this problem has been to buffer the data in the control unit controlling the I/O device. The amount of buffering required can become considerable whenever there is a significant difference in data rates.
The buffering approach can be expensive and wasteful of system resources. In the present invention there is a limited amount of buffering, one stage, and the control unit for the I/O device is so structured that it groups cycle steal requests and relinquishes a cycle steal between groups whereby controls are switched to enable the granting of this relinquished cycle steal to another I/O device. Thus, the present invention reduces the amount of buffering which would otherwise be necessary and also frees up system resources for another I/O device which may be wanting service and thereby improving the overall efficiency of the computer system.